1. Technical Field
The present invention relates to an ignition coil applicable to an internal combustion engine.
2. Related Art
An ignition coil used in an internal combustion engine, such as an engine, includes a primary coil, a secondary coil, a center core, and the like. The primary coil and the secondary coil are disposed concentrically. The center coil is disposed in an axial-center position of the primary coil and the secondary coil. Constituent components, such as the primary coil, the secondary coil, and the center coil, are housed within a case. Gaps formed in the case are filled with a thermoset resin, such as an epoxy resin.
For example, JP-A-2003-92225 discloses an ignition coil. In the ignition coil, a connection terminal composed of an elastic body is connected to a high-voltage-side winding end portion of the secondary coil. A high-voltage terminal is press-fitted into a high-voltage tower section. The high-voltage tower section has a cylindrical shape and is formed projecting from the case. In the ignition coil, during assembly of the constituent components, such as the secondary coil, into the case, the connection terminal and the high-voltage terminal are placed in contact in an axial direction of the high-voltage tower section. As a result, electrical conduction is ensured between the connection terminal and the high-voltage terminal.
However, the above-described ignition coil in JP-A-2003-92225 has the following issues. In other words, when the constituent components, such as the primary coil, the secondary coil, and the center core, are assembled into the case, the state of contact between the connection terminal and the high-voltage terminal is ensured by contact pressure caused by the elastic force of the connection terminal that is composed of an elastic body. However, after the case is filled with the thermoset resin, such as an epoxy resin, and the thermoset resin is hardened, the connection terminal is confined by the thermoset resin. Therefore, exerting the elastic force of the connection terminal becomes difficult.
In a state such as this, there is an instance in which the linear thermal expansion coefficient of the material configuring the case is greater than the linear thermal expansion coefficient of the thermoset resin, such as an epoxy resin. In this instance, when the ignition coil is mounted in an internal combustion engine and used in a high-temperature environment, the difference in linear thermal expansion coefficient causes displacement of the high-voltage terminal. Specifically, the high-voltage terminal that is press-fitted into the high-voltage tower becomes displaced towards a direction away from the connection terminal in the axial direction of the high-voltage tower.
In addition, there is an instance in which the linear thermal expansion coefficient of the material configuring the case is less than the linear thermal expansion coefficient of the thermoset resin, such as an epoxy resin. In this instance, when the ignition coil is mounted in the internal combustion engine and used in a low-temperature environment, the connection terminal becomes displaced towards a direction away from the high-voltage terminal in the axial direction of the high-voltage tower section.
As a result, even the slightest amount of displacement causes contact failure between the connection terminal and the high-voltage terminal. Electrical conduction between the connection terminal and the high-voltage terminal becomes difficult to ensure. Ignition energy loss and noise increase caused by connection due to micro-discharge, breakdown caused by contact failure, and the like may occur.